This invention relates to moving coil loudspeakers of the type including a pair of co-axial speech coils, one of which drives a high frequency diaphragm at the rear of the loudspeaker to provide an output along a horn and the other of which drives a cone forming a continuation of the horn and reproducing the lower frequencies. An example of such a construction is described and illustrated in British patent specification No. 893,838.
The angle of divergence of a horn is characteristically narrow where the wavelength of the radiated sound is small compared with the horn mouth circumference. As a result over some areas a listener will not hear higher frequencies at the same strength as lower frequencies. This is detrimental to the acoustic reproduction of sounds, and is a well known drawback of this type of loudspeaker construction.
According to the present invention, an acoustic lens is fitted in the region where the horn contour is taken over by the cone, the lens being shaped over its external surface so as to conform snugly with the contour of the cone and being effective to increase the divergence of the beam of acoustic radiation from the horn in the operative plane of the lens. Since, as mentioned above, the cone forms an extension of the horn, the acoustic lens is thus situated within the outer part of the horn. As is well known, such a lens operates by increasing the path length of peripheral radiation in relation to that closer to the axis, thus increasing the curvature of the wave front and hence the divergence of the beam of acoustic radiation in the plane for which the lens is designed and which, in practice, will normally be the horizontal plane. The lens may thus be designed to give any required degree of divergence to the acoustic beam from the horn and this may conveniently match that from the cone.
The acoustic lens is preferably of the slant-plate type, but other types of such lens may be used such as the perforated plate type, for example. Whatever the type of lens, it is conveniently held in position by a cruciform mounting extending across the mouth of the horn proper.